The present invention relates to a getter pump especially suitable for the use upstream, in proximity and coaxially with respect to a turbomolecular pump.
The getter pumps are static pumps, i.e. lack mechanical moving members, and their working is based on the chemisorption of reactive gases such as oxygen, hydrogen, water and carbon oxides by elements made of non-evaporable getter materials (known in the field as NEG materials). The main NEG materials are alloys based on zirconium or titanium.
The getter pumps for generating and keeping the high vacuum in an enclosed environment nearly always work combined with other pumps; in particular, the first high-pressure pumping stage is performed by mechanical pumps such as rotary or diffusion pumps, whereas getter pumps combined with chemical-ion, cryogenic or turbomolecular pumps may be used for attaining high vacuum.
It is especially advantageous to combine getter pumps with turbomolecular pumps. In fact, the efficiency of turbomolecular pumps decreases upon decreasing of the molecular weight of the gas and therefore their efficiency is low for hydrogen, which is one of the gases mainly contributing to the residual pressure in evacuated systems in the medium vacuum range and is the main residual gas at pressures lower than 10.sup.-9 hPa. On the other hand, the getter pumps are especially effective in pumping hydrogen, in particular for temperatures ranging from room temperature to about 300 .degree. C. Thus the combination of a getter pump and a turbomolecular pump, in that combining different behaviors with respect to the gases present in the system or anyhow to remove, is an optimal solution for the problem of evacuating a chamber. In particular, this combination is advantageous in case the chamber to be evacuated is a working chamber used for high-vacuum operations, such as e.g. a chamber of a process machine of the semi-conductor industry.
These advantages are in principle maximized when the two pumps are arranged in series, with the getter pump being upstream with respect to the turbomolecular pump. However, so far the two pumps have never been arranged in series, but have always been mounted through flanges onto two different openings of the chamber to be evacuated, in order to avoid the following problems and drawbacks:
the getter elements forming the pump are generally produced by compacting NEG material powders; the getter pump is thus liable to loose particles possibly hitting the turbomolecular pump blades and damaging them, or causing the pump to grip by coming between its rotor and its stator; PA1 interposing a getter pump between the chamber to be evacuated and the turbomolecular pump generally results in a decrease of the gas conductance to this latter; PA1 when the getter pump is working, the non-evaporable getter material must be kept at temperatures of about 200-300.degree. C.; for this purpose it was so far heated by irradiation from inside the pump by means of lamps or filament resistances wound upon a generally ceramic support, or from outside the pump by means of suitable heating members arranged on the pump body; thus, a rise of the turbomolecular pump temperature might also occur resulting in expansion of the blades beyond the tolerances (being moreover very small) acceptable for a good pump working. On the other hand, the increase of the distance between the pumps or the incorporation of thermal shields therebetween in order to reduce the effect of the rise of the turbomolecular pump temperature would result in an unacceptable reduction of the gas flow conductance.
Another drawback, however less important than those indicated above, was the fact that, by using the aforementioned heating systems, thermocouples had to be necessarily provided on the getter pump for measuring the temperature of the active material whereby complex tightness problems related to the wires having to come out from a vacuum-environment had to be solved.